1. Field of the Invention
The present invention relates to an induction heating cooker that employs an inverter circuit over inductively heating an object, and particularly to an induction heating cooker of large input power that causes no noise from its power source, achieves excellent efficiency and is capable of continuously changing its input power for a wide range.
2. Description of the Prior Art
An induction heating cooker produces no flame, and therefore, is safe and achieves excellent heating efficiency.
FIG. 1 is a block circuit diagram showing a conventional induction heating cooker employing an inverter circuit 104 of the quasi-E class. An input setting circuit 118 sets an input value according to which a PWM oscillator 116 provides a pulse signal. According to the pulse signal, a driving circuit 114 sets an ON time TON for a transistor 112. The transistor 112 is turned on and off in response to pulse signals from the driving circuit 114 to put a heating coil 106 and a resonant capacitor 108 in a series resonant state. Accordingly, the heating coil 106 generates magnetic flux, which causes an electromagnetic induction action to generate an eddy current in an object (not shown) such as a pan. As a result, the object is heated. An advantage of the inverter circuit 104 of quasi-E class is that high-frequency electric power can be generated with a single switching element (the transistor 112).
If the input power is increased, a resonance voltage VCE is increased as shown in FIG. 2a. The high resonance voltage is critical to a withstand voltage of the switching element (transistor 112). To reduce the input power as shown in FIG. 2b, the ON time TON of the transistor 112 shall be shortened. In this case, the transistor 112 is usually turned on before the resonance voltage VCE reaches zero volts. If this happens, an excessive short-circuit current IS flows to the transistor 112 to destroy the transistor.
Supposing the cooker is 200 V in power source voltage and 2 KW in maximum input power, the resonance voltage VCE will reach 1100 V for the maximum input power. When the ON time TON of the switching element is reduced to bring the input power to 1 KW, the magnitude of the short-circuit current will be 80 A.
Supposing the cooker is 3 KW in maximum input power, the resonance voltage VCE will be 1800 V for the maximum input power. To bring the input power below 2 KW, the short-circuit current IS must be very large. To avoid this, it is necessary to repeatedly turn on and off the inverter circuit. This may, however, change the temperature of the cooker and deteriorate cooking efficiency.
If the maximum input power is 3.5 KW to shorten the cooking time, the resonance voltage VCE may reach 2000 V or over. There is no such switching element that can withstand the resonance voltage of 2000 V and achieve a high-speed switching operation. The inverter circuit of quasi-E class is, therefore, not applicable for a large power induction heating cooker.
For such a large power induction heating cooker, a bridge inverter circuit has been proposed. In this type of cooker, a voltage larger than a power source voltage is applied to its switching element so that input power of the cooker may easily be increased. In addition, the cooker can heat an object made of non-magnetic material such as aluminum and stainless steel.
To control the input power of the cooker, the bridge inverter circuit is turned on and off. Alternatively, as shown in FIG. 3, an input controlling circuit 133 may provide a control signal based on which thyristors 107a and 107b are controlled, thereby continuously controlling the input power. This technique is called phase control.
In FIG. 3, a half bridge inverter circuit 125 receives signals from an inverter driving circuit 113 to alternately turn transistors 115 and 117 on and off, thereby applying high-frequency electric power to a heating coil 119.
A conventional induction heating cooker employing the bridge inverter circuit that is turned on and off to control input power has a problem of generating a repulsive force in heating an aluminum pan. As shown in FIG. 5, heating the aluminum pan with a cooker of 2000 W in input power generates a repulsive force of 920 g. If the aluminum pan weighs, for example, about 1 Kg, the pan may move over a top plate of the cooker. This is dangerous. If the bridge inverter circuit is turned on and off to decrease the input power from 2000 W, a replusive force of 920 g is intermittently generate whenever the inverter circuit is turned on, to gradually move the aluminum pan and generate unpleasant noise.
In FIG. 3, the input power is continuously controlled, and an input current IIN from an AC power source 101 is intermittently supplied, as shown in FIGS. 4a and 4b. Due to this, the power source emits noise.
To deal with this, a large capacity reactor 103 is inserted between the AC power source 101 and the bridge circuit 105. The reactor or a thyristor, however, has a loss that lowers efficiency.
A thyristor, if employed, requires a radiating plate, which raises another problem of increasing the size of the cooker.